it depends. If I am diving backmount with DIR staged bottles, then it's all on the left. If I am diving my backplate that has bungees on it, then lean left, right rich for deco bottles. All stage bottles go on the left regardless. If bottles are stowed, then they'll be behind me, with O2 coming up to the right side when I'm ready for it.
in sidemount I dive with stages on both sides and have not had an issue with DPV use. That said, those bottles are staged, and are tucked in a lot tighter than the standard DIR rigging that leaves bottles hanging quite loose at the neck. My long hose donation is usually not a concern for me because in an OOA situation on stages, it should take quite a while for a diver to come at you, but if he does, then you are giving him the stage reg first and going to your secondary. That bottle is then given to the OOA diver and you head on your merry way. If you are on the left stage, you then switch to the right and that one gets donated if another OOA shows up.
By the time you get to the point that you are donating the long hose, you don't have anything left as stage bottles since it is always primary donate.
I put my O2 bottle on the right if I can help it. If I can't leave it there during the dive, i.e. not staging it, then it stays on the left in backmount. In sidemount it doesn't matter so stays on the right since the long hose is snaked in the bungee on the tank itself vs. looped around my body.
of note, I dive Jetstreams which are non-directional. If diving with "normal" regulators, then there are pros and cons. I know a few guys that use lefty regs on their O2 bottles for added differentiation in zero vis. If running on the right, it loops around the same as any other bottle just as a mirror image. If on the left then they just don't loop around your neck. I'm not a huge fan of that personally, but to each his own
